Compound semiconductors have been applied to semiconductor devices such as solar cells and light emitting elements. When such devices are manufactured, a p-type semiconductor and an n-type semiconductor are necessary and compound semiconductors are doped with elements other than the constituent elements, in order to obtain semiconductors of respective conduction types.
It has been known, for example, in a case of Ib-IIIb-(VIb).sub.2 group compound semiconductors such as CuInS.sub.2 to add impurity of a Vb group element for obtaining a p-type and a VIIb group element for obtaining an n-type.
Further, for IIIb-vb group compound semiconductors such as GaN, a IIa group element is used for obtaining the p-type and a IVb group element is used for obtaining the n-type, respectively, as impurities and, for IIb-VIb group compound semiconductors such as ZnSe, a Vb group element is used for obtaining the p-type and a VIIb group element is used for obtaining the n-type, respectively, as impurities.
Such compound semiconductors show conductivity (p-type or n-type) by substitution of an added impurity for a portion of constituent elements of the compound semiconductor. When a p-type semiconductor is formed, generally, an element with the number of electrons in the outermost shell smaller by one compared with that of a constituent element to be substituted is chosen as the impurity, whereas an element with the number of electrons in the outermost shell larger by one is chosen in a case of an n-type semiconductor.
However, when a p-type impurity is doped to a compound semiconductor for obtaining the p-type conductivity, particularly, a compound semiconductor having a strong ionic bond character, a negative element (electron receptive element) constituting the compound semiconductor is eliminated and the acceptor for the valence electron of the positive element (electron donating element ) is lost to generate a donor defect. The effect of the doped p-type impurity is offset by the donor defect, so that the p-type carrier concentration is not increased as expected. That is, there is a problem, in that it is difficult to obtain a p-type semiconductor at a high carrier concentration corresponding to a high doping amount.
Further, if the amount of the added p-type impurity is increased greatly in order to increase the carrier concentration, this causes a lowering of the crystallinity due to crystal distortions in a compound semiconductor as a matrix or segregation of a doping element to a crystal grain boundary, and this results in a problem of failing to obtain a necessary function.
With the problems described above, semiconductor devices utilizing p-type semiconductors having compound semiconductors with strong ionic bonds as the matrix, for example, those having satisfactory performance as solar cells based on CuInS.sub.2, or the like and blue light emitting elements based on GaN, ZnSe or CuAlSe.sub.2, have not yet been obtained.
Among them, for a p-type semiconductor having GaN as the matrix, it has been reported by Brandt, et al that a p-type semiconductor having a high carrier concentration can be formed by adding Be as an acceptor, together with 0 as a donor, to GaN (refer to Appl. Phys. Lett., Vol. 69, No. 18 (1996) 2707). In this case, since the difference of the electric negativity between impurities Be and 0 is large and, they tend to be present in the vicinity of GaN crystals, the carrier concentration is increased.
Further, Yamamoto, et al. propose to supply electrons to Cu-S bonds by the doping of a p-type impurity by partially substituting In atoms for Cu atoms as the constituent elements of CuInS.sub.2, thereby increasing the p-type carrier concentration of CuInS.sub.2, (See "The 1996 MRS Spring meeting Symposium in San Francisco").
That is, when Cu atoms of a Ib group element are partially substituted with In atoms of a IIIb group element, to provide a state: Cu&lt;In, the number of electrons in CuInS.sub.2 is increased compared with that in the state of Cu:In=1:1, and surplus electrons are supplied to the Cu--S bonds. But this proposed method is different from the method of doping CuInS.sub.2 with a p-type impurity and an n-type impurity, which are not constituent elements of CuInS.sub.2.
However, when Cu atoms as the constituent elements of CuInS.sub.2 are partially substituted with In atoms, InCu (In substituted for the site of Cu) is formed as a donor defect and, at the same time, a Cu vacancy is formed as an acceptor defect for keeping electric charges neutral in the vicinity thereof. Accordingly, since it is difficult to exactly control the amount of electrons supplied to the Cu--S bonds by the substitution amount of In for Cu, this method involves a difficulty in the manufacture of the devices.
Further, since In atoms become excessive in this method, a CuIn.sub.5 S.sub.8, phase having a spinel structure, which is different from CuInS.sub.2 having a chalcopyrite structure, is also present with the CuInS.sub.2. If phases of different crystal structures are present together, many defects tend to occur, particularly, at the crystal grain boundaries of the CuInS.sub.2 layer. In addition, the CuIn.sub.5 S.sub.8, phase has high resistivity. As a result, it also gives rise to a problem of difficulty in forming a p-type CuInS.sub.2, at a high carrier concentration.
The present invention has been accomplished taking notice on the foregoing problems in the prior art and it is an object thereof to form a p-type semiconductor at a high carrier concentration in an Ib-IIIb-VIb.sub.2 group compound semiconductor such as CuInS.sub.2, which has higher performance and gives more advantage in the manufacture thereof compared with the prior art method, and to provide a semiconductor device of higher performance by using such p-type semiconductor.